The analysis of complex mixtures such as biological fluids by gas chromatography often requires the use of selective detectors to discriminate against interfering species and/or enhance the response to desired compounds. However, with the exception of mass spectrometers, most GC detectors are either difficult or impossible to operate in a wide variety of selective and nonselective modes. Although not designed to provide the range and scope of data provided by mass spectrometers, the ion mobility spectrometer is capable of quantitative and low resolution qualitative detection organic compounds. When compared with commonly employed selective detectors such as the electron capture and nitrogen-phosphorus detectors, the ion mobility spectrometer offers the distinct advantage of tunable selectivity. Because of its versatility and low cost when compared to that of mass spectrometers, the instrument has great potential for routine screening of many drugs in biological samples.